Variable speed transmission for a rotary wing aircraft

ABSTRACT

A transmission gearbox for a rotary-wing aircraft includes a main gearbox and a variable speed gearbox in meshing engagement with the main rotor gearbox. The variable speed gearbox permits at least two different RPMs for the main rotor system without disengaging the engine(s) or changing engine RPMs. The variable speed gearbox includes a clutch, preferably a multi-plate clutch, and a freewheel unit for each engine. A gear path drives the main gearbox in a “high rotor speed mode” when the clutch is engaged to drive the main rotor system at high rotor rpm for hover flight profile. A reduced gear path drives the main gearbox in a “low rotor speed mode” when the clutch is disengaged and power is transferred through the freewheel unit, to drive the main rotor system at lower rotor rpm for high speed flight. The variable speed gearbox may be configured for a tail drive system that operates at a continuous speed, a tail drive system that changes speed with the main rotor shaft or for no tail drive system.

BACKGROUND OF THE INVENTION

The present invention relates to a rotary-wing aircraft, and moreparticularly to a rotary wing transmission gearbox system which providesvariable speeds to facilitate high speed and low speed flight profiles.

The forward airspeed of a conventional rotary wing aircraft is limitedby a number of factors. Among these is the tendency of the retreatingblade to stall at high forward airspeeds. As the forward airspeedincreases, the airflow velocity across the retreating blade slows suchthat the blade may approach a stall condition. In contrast, the airflowvelocity across the advancing blade increases with increasing forwardspeed. Dissymmetry of lift is thereby generated as forward air speedincreases.

This dissymmetry of lift may create an unstable condition if notequalized across the advancing and retreating sectors of the rotor disc.Typically, blade flapping and feathering are utilized to substantiallyequalize the lift.

However, as the forward airspeed is increased beyond a given point for agiven rotor rpm, the flapping and feathering action eventually becomesinadequate to maintain substantial equality of lift over the rotor disc.At this point, reverse airflow across the retreating sector createsnegative lift and, depending on the forward speed, creates a stalling ornegative lift condition that travels outwardly across the blade asairspeed increases. Conventional rotors must be operated at airspeedslower than those which cause reverse airflow across a substantial partof the retreating blade and at an rpm low enough to alleviate anypotential compressibility Mach number problems at the tip of theadvancing blade. This has effectively limited forward airspeeds ofconventional helicopters to approximately 180 knots.

A rotary wing aircraft with a coaxial counter-rotating rigid rotorsystem is capable of higher speeds as compared to conventional singlerotor helicopters partly due to the balance of lift between theadvancing sides of the main rotor blades on the upper and lower rotorsystems. In addition, the retreating side of the rotor discs are alsogenerally free from classic retreating blade stall due to offloading ofthe retreating disc sector with increasing airspeed to obtain rollequilibrium by balancing the net effects of the equal and oppositemoments produced by the advancing sectors of the upper and lowercounter-rotating rotor systems. To still further increase airspeed, acompound rotary wing aircraft may incorporate supplemental translationalthrust.

In high speed flight, the main rotor system may be unloaded from theturbojets, and rotor RPM may be controlled by adjusting collectivepitch. For any rotary-wing aircraft, increasing collective pitch slowsthe rotational speed and decreasing collective pitch increasesrotational speed. For a rotary wing aircraft in a high speed flightprofile, however, rotor RPM needs to be decreased to prevent the rotorblade tips on the advancing sides of the rotor discs from entering asupersonic region as aircraft airspeed increases. Thus, as forwardairspeed increases, collective pitch must be increased to prevent therotor RPM from increasing to an undesirable level. However, as theforward airspeed is increased beyond a given point for a given rotorrpm, adjusting collective pitch eventually becomes inadequate.

The aerodynamics of high-speed rotary wing aircraft show a noticeablebenefit by reducing rotor RPM in high speed cruise flight. The RPMreduction from a hover profile to a high speed flight profile istypically on the order of about 30%. Such a reduction at the engine,however, may cause problems with auxiliary systems, engine operation andavailable power while, a rotary-wing aircraft, which always operates ata relatively low rotor RPM, may present penalties in rotor andtransmission weight, as well as maneuverability constraints. Thus, thereis a need for a rotary-wing transmission gearbox system, which providesvariable rotor system speeds.

Accordingly, it is desirable to provide a variable speed gearbox systemfor a rotary-wing aircraft that provides a “high rotor speed mode” and“low rotor speed mode” to maximize aircraft performance during a hoverflight profile and a high speed cruise flight profile, respectively.

SUMMARY OF THE INVENTION

A transmission gearbox system of a rotary-wing aircraft according to thepresent invention includes a main gearbox and a variable speed gearboxin meshing engagement with the main gearbox. The variable speed gearboxpermits at least two different main rotor system RPMs withoutdisengaging the engines or changing engine RPM. The variable speedgearbox facilitates different flight spectrums, such as a hover flightprofile and a high speed cruise flight profile for any rotary wingaircraft. Typically during landing, take-off, hover and low speed flightprofiles, a higher main rotor speed is required for increased liftingcapabilities while in a high speed cruise flight profile a lower mainrotor speed is desired for improved rotor performance and higher forwardairspeed.

The variable speed gearbox includes a clutch, preferably a multi-plateclutch, and a freewheel unit for each engine. In a “high rotor speedmode,” when the clutch is engaged to drive the main rotor system at highrotor rpm, a gear path drives the main gearbox in a direct drive (1 to 1ratio). For a high speed cruise flight profile with a lower rotor RPM ina “low rotor speed mode,” the clutch is disengaged and power istransferred through a reduced gear path, preferably by way of atwo-stage gear reduction, a freewheel unit and into the main rotorgearbox.

The variable speed gearbox may be configured for a tail drive systemthat operates at a continuous speed, a tail drive system that changesspeed with the main rotor shaft or with no tail drive system. Foraircraft models that require a continuous tail speed, the tail drivesystem may be in meshing engagement with the input side of the clutchand freewheel unit. For aircraft models that would benefit from a taildrive system which changes speed commensurate with the main rotorsystem, the tail drive system may be in meshing engagement with theoutput side of the clutch and freewheel unit or directly to the maingearbox to maintain a fixed gear ratio relative to the engine.

The present invention therefore provides a variable speed gearbox systemfor a rotary-wing aircraft that provides a “high rotor speed mode” and“low rotor speed mode” to maximize aircraft performance during a hoverflight profile and a high speed cruise flight profile, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows:

FIG. 1A is a general phantom side view of an exemplary rotary wingaircraft embodiment for use with the present invention;

FIG. 1B is a general phantom top view of the exemplary rotary wingaircraft embodiment of FIG. 1A;

FIG. 1C is a general side views of a rotary wing aircraft with ananti-torque tail rotor embodiment for use with the present invention;

FIG. 2A is a block diagram of a transmission gearbox system of thepresent invention;

FIG. 2B is a block diagram of an alternative transmission gearbox systemof the present invention; and

FIG. 2C is a block diagram of an alternative transmission gearbox systemof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1A and 1B illustrate a vertical takeoff and landing (VTOL)rotary-wing aircraft 10 having a main rotor system 12. The main rotorsystem 12 is preferably a dual, counter-rotating, coaxial rotor system,however, any other rotor system known in the art including, but notlimited to, single, tandem and dual rotor systems may also be used withthe present invention. That is, although the present invention is beingdescribed in combination with a high speed compound rotary wingaircraft, other aircraft configurations, including a more conventionalconfiguration with a single main rotor system 12′ and an anti-torquetail rotor system 32′ (FIG. 1C) will also benefit from the presentinvention.

As shown, the aircraft 10 includes an airframe 14 which supports themain rotor system 12. The aircraft 10 may also incorporate a tail drivesystem 30. The tail drive system 30 is preferably a translational thrustsystem 32 that provides translational thrust generally parallel to anaircraft longitudinal axis L.

The main rotor system 12 preferably includes a first rotor system 16 anda second rotor system 18. Each rotor system 16, 18 includes a pluralityof rotor blades 20 mounted to a rotor hub 22, 24 for rotation about arotor axis of rotation R. A drive system 35 drives the main rotor system12. The translational thrust system 32 preferably includes a pusherpropeller 34 having a propeller rotational axis P oriented substantiallyhorizontal and parallel to the aircraft longitudinal axis L to providethrust for high-speed flight. Preferably, the pusher propeller 32 ismounted within an aerodynamic cowling 36 mounted to the rear of theairframe 14. The translational thrust system 32 is preferably driven bythe same drive system 35 which drives the main rotor system 12.

Referring to FIG. 2A, the drive system 35 of the aircraft 10 isschematically illustrated. A gearbox 26 is preferably interposed betweenone or more gas turbine engines (illustrated schematically at E), themain rotor system 12 and the translational thrust system 32. Preferably,the gearbox 26 includes a main gearbox 38 and a variable speed gearbox40 in meshing engagement with the main gearbox 38. For furtherunderstanding of a main gearbox and associated components thereof, whichmay be used in connection with the present invention, attention isdirected to U.S. patent application Ser. No. 11/140,762 entitled SPLITTORQUE GEARBOX FOR ROTARY WING AIRCRAFT WITH TRANSLATIONAL THRUST SYSTEMwhich is assigned to the assignee of the instant invention and which ishereby incorporated by reference in its entirety.

Each engine E preferably drives the variable speed gearbox 40 through anengine freewheel unit 42 as generally understood by one of ordinaryskill in the art to permit single engine operation and autorotationshould all engines fail. Although only the gear train from engine #1will be discussed in detail herein, the gear train from engine #2 isidentical and it should be understood that any number of engines E maybe utilized with the present invention.

The variable speed gearbox 40 of the present invention permits at leasttwo different rotor speeds for the main rotor system 12 withoutdisengaging the engines E or changing engine RPM. The variable speedgearbox 40 facilitates different flight profiles, such as a low speedflight profile and a high speed flight profile for any rotary wingaircraft. Typically during landing, take-off, hover and low speed flightprofiles, a higher main rotor speed is required for increased liftingcapabilities while in a high speed cruise flight profile, a lower mainrotor speed is desired for improved rotor performance and increasedairspeed.

The variable speed gearbox 40 includes a clutch, preferably amulti-plate clutch 44, and a freewheel unit 46 for each engine E. Themulti-plate clutch 44 and the freewheel unit 46 are driven in parallel.A high speed input shaft 48 from the engine freewheel unit 42 drives thevariable speed gearbox 38 primarily through a gear path 50, (illustratedschematically as shafts 48, 44 i, and 44 o) when the clutch 44 isengaged and primarily through a reduced gear path 52, preferably througha two-stage gear reduction 52 a, 52 b (reduced gear path 52 illustratedschematically herein as the gear path of 48, 52 a, 46 i, 46 o, 52 b)when the clutch 44 is disengaged. It should be understood that variousgear ratios will be usable with the present invention and that “gearpath” as discussed herein is with reference to a first gear path whichdrives the main rotor system 12 in a direct drive which includes aclutch and that “reduced gear path” as discussed herein is withreference to a second gear path which includes a freewheel unit and agear reduction, but are not otherwise limiting.

The variable speed gearbox 40 preferably drives the main gearbox 38 in adirect drive (1 to 1 ratio) “high rotor speed mode” when the clutch 44is engaged to drive the main rotor system 12 at high rotor rpm. In thismode, a freewheel output 46 o of the freewheel unit 46 is rotatingfaster than a freewheel input 46i of the freewheel unit 46, so that thefreewheel unit 46 overruns and does not transmit power. A relativelylower torque at relatively higher RPM is thereby transmitted for a givenengine horsepower.

For the high speed cruise flight profile with a lower rotor RPM in a“low rotor speed mode,” the clutch 44 is disengaged and power istransferred preferably from the input shaft 48, to an initial gearreduction 52 a through the freewheel unit 46 to another gear reduction52 b, and into the main gearbox 38. It should be understood that “highrotor speed mode” and “low rotor speed mode” are utilized herein only asrelative terms regarding the speed of the main gearbox 38 due tooperation of the variable speed gearbox 40 and should not be consideredotherwise limiting. With the clutch 44 disengaged, a clutch input 44 iof the clutch 44 is spinning at the same speed as the engine E and aclutch output 44 o of the clutch 44 is spinning at a reduced speed as aresult of the gear reduction as connected by the freewheel unit 46. Arelatively higher torque at a lower RPM is thereby transmitted for agiven engine horsepower.

Notably, both the gear path 50 and the reduced gear path 52 continue torotate irrespective of engagement/disengagement of the clutch 44 so thatthe clutch 44 and freewheel unit 46 need only accommodate the relativedifference in speed and inertia between the gear ratios of the gear path50 and the reduced gear path 52. This provides a lightweight and rapidreacting system compared to a conventional transmission which mustaccelerate or decelerate part of the drive system from or to zero speed.

The gearbox configuration of the present invention provides alightweight system while assuring operation in the event of a clutchseizure or complete clutch slip. In the event of a clutch seizure, poweris still transmitted from the engine to the main gearbox. In the eventof a clutch slipping, power is automatically diverted through thefreewheel unit to the main gearbox.

Transition from “low rotor speed mode” back to the “high rotor speedmode” such as when an aircraft is decelerating from cruise flight to ahover, requires engaging the clutch 44 and allowing the main rotor speedto increase until the clutch output 44 o matches the speed of the clutchinput 44 i and the clutch 44 fully locks. The clutch 44 may be operatedin an on/off manner or in a feathering manner in whichengagement/disengagement is gradually applied.

Transitions between the “low rotor speed mode” and the “high rotor speedmode” are alternatively assisted through aircraft flight controlmethods. Preferably, main rotor speed is increased or decreased viarotor collective pitch control and aircraft flight attitude. That is, tochange the amount of lift generated by the rotor system, either thespeed and/or the blade angle of attack may be changed. If the angle ofattack and the speed are changed in such a way that those changes canceleach other out, the same amount of lift is produced. Therefore, whentransitioning from “low rotor speed mode” to “high rotor speed mode,”collective pitch is modulated to lower the angle of attack of the bladesas the rotor speed increases to provide the same amount of liftthroughout the transition. In addition, when the angle of attack isdecreased, the amount of drag produced is also decreased which increasesrotor speed for equivalent power. For the transition from a “high rotorspeed mode” to a “low rotor speed mode,” the opposite generally occurs.With aircraft flight control methods, transitions are rapidlyfacilitated with less inertia loads applied to the clutch.

The variable speed gearbox 40 may also be configured for a tail drivesystem 30 that operates at a continuous speed (FIG. 2A), a tail drivesystem that changes speed with the main rotor system 12 (FIG. 2B) orwith no tail drive system 30 (FIG. 2C). FIGS. 2A and 2B are preferablyutilized with either a high speed compound rotary wing aircraft 10having an optional translational thrust system 32 (FIG. 1A and 1B) orthe more conventional aircraft 10′ configurations (FIG. 1C) with ananti-torque tail rotor system 32′.

The variable speed gearbox 40 may drive the tail drive system 30 througha tail drive gear reduction 54 in meshing engagement therewith. Foraircraft that require a continuous tail speed, the tail drive system 30of FIG. 2A is preferably utilized in which the tail drive system is inmeshing engagement with the input side of the clutch 44 and freewheelunit 46 to thereby maintain a fixed gear ratio relative the engine E.For aircraft that would benefit from a tail drive system 30 whichchanges speed commensurate with the main rotor system 12, the tail drivesystem 30 of FIG. 2B is preferably utilized in which the tail drivesystem is in meshing engagement with the output side of the clutch 44 oand freewheel unit 46 o or directly to the main gearbox 26 to therebymaintain a fixed gear ratio relative the main rotor shaft.

The choice of tail drive system 30 configuration is determined byseveral factors including tail rotor efficiency, noise requirements,performance objectives, etc. The amount of force the anti torque tailrotor is required to generate in order to counteract the torque of themain rotor must change when the main rotor speed changes, and as aresult torque, changes. To compensate for a change in main rotor torque,the tail rotor can either change its angle of attack and maintain thesame speed (FIG. 2A), maintain the same angle of attack and change speed(FIG. 2B), or change both (FIG. 2B). The FIG. 2B configuration mayalternatively be utilized with a compound helicopter where the tailrotor changes orientation to provide force to counteract the main rotortorque in hover and then transition to a pusher propeller orientation toprovide translational thrust during a high speed flight profile.

It should be understood that relative positional terms such as“forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like arewith reference to the normal operational attitude of the vehicle andshould not be considered otherwise limiting.

It should be understood that although a particular component arrangementis disclosed in the illustrated embodiment, other arrangements willbenefit from the instant invention.

Although particular step sequences are shown, described, and claimed, itshould be understood that steps may be performed in any order, separatedor combined unless otherwise indicated and will still benefit from thepresent invention.

The foregoing description is exemplary rather than defined by thelimitations within. Many modifications and variations of the presentinvention are possible in light of the above teachings. The preferredembodiments of this invention have been disclosed, however, one ofordinary skill in the art would recognize that certain modificationswould come within the scope of this invention. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described. For thatreason the following claims should be studied to determine the truescope and content of this invention.

1. A method of controlling a speed of a main rotor system of arotary-wing aircraft comprising the steps of: (1) engaging a clutchdriven in parallel with a freewheel unit, the clutch, and the freewheelunit driven by a single engine; (2) overrunning a freewheel output overa freewheel input of a freewheel unit in response to said step (1) todrive a main gearbox of the main rotor system at a first speed; (3)disengaging the clutch; and (4) running a clutch input separately from aclutch output in response to said step (3) to drive the main gearbox ofthe main rotor system at a second speed through the freewheel unit.
 2. Amethod as recited in claim 1, wherein said step (4) further comprises:(a) driving an output of a gear path and an output of a reduced gearpath at a speed equivalent to the freewheel output to drive the maingearbox of the rotor system at the second speed.
 3. A method as recitedin claim 1, wherein said step (4) further comprises: (b) driving theclutch output at a speed equivalent to the output of the gear path, theoutput of the reduced gear path and the freewheel output.
 4. A method asrecited in claim 1, wherein said step (4) further comprises: (b) drivinga clutch input at a speed equivalent to an engine.
 5. A method asrecited in claim 1, wherein said step (4) further comprises: (a) drivingthe freewheel output by the freewheel input.
 6. A method as recited inclaim 1, further comprising the step of: (5) driving a tail drive outputat a constant speed through a tail drive gear reduction in meshingengagement with the freewheel input of the first freewheel unit.
 7. Amethod as recited in claim 6, wherein said step (5) further comprises:(a) driving a translational thrust system with the tail drive output. 8.A method as recited in claim 6, wherein said step (5) further comprises:(a) driving an anti-torque rotor with the tail drive output.
 9. A methodas recited in claim 1, further comprising the step of: (5) driving atail drive output at a variable speed through a tail drive gearreduction in meshing engagement with the freewheel output of the firstfreewheel unit.
 10. A method as recited in claim 9, wherein said step(5) further comprises: (a) driving a translational thrust system withthe tail drive output.
 11. A method as recited in claim 9, wherein saidstep (5) further comprises: (a) driving an anti-torque rotor with thetail drive output.
 12. A method as recited in claim 9, wherein said step(5) further comprises: (a) driving an anti-torque rotor with the taildrive output.
 13. A method as recited in claim 9, wherein said step (1)further comprises: (a) driving the first and second engine in parallel.14. A method as recited in claim 9, wherein said step (1) furthercomprises: (a) driving the first clutch and first freewheel output inparallel with the second clutch and the second freewheel unit.
 15. Amethod as recited in claim 14, wherein said step (1) further comprises:(b) driving the first and second engine in parallel.
 16. A method ofcontrolling a speed of a main rotor system of a rotary-wing aircraftcomprising the steps of: (1) engaging a first clutch driven in parallelwith a first freewheel output, the first clutch and the first freewheelunit driven by a first engine and engaging a second clutch driven inparallel with a second freewheel output, the second clutch and thesecond freewheel unit driven by a second engine; (2) overrunning thefirst and second freewheel output over a first and second freewheelinput of the respective first and second freewheel unit in response tosaid step (1) to drive a main gearbox of the main rotor system at afirst speed; (3) disengaging the first and second clutch; and (4)running a first and second clutch input separately from a first andsecond clutch output in response to said step (3) to drive the maingearbox of the main rotor system at a second speed through the first andsecond freewheel unit.
 17. A method as recited in claim 16, furthercomprising the step of: (5) driving a tail drive output at a constantspeed through a tail drive gear reduction in meshing engagement with thefirst and second freewheel input.
 18. A method as recited in claim 17,wherein said step (5) further comprises: (a) driving a translationalthrust system with the tail drive output.
 19. A method as recited inclaim 17, wherein said step (5) further comprises: (a) driving ananti-torque rotor with the tail drive output.
 20. A method as recited inclaim 19, further comprising the step of: (5) driving a tail driveoutput at a variable speed through a tail drive gear reduction inmeshing engagement with the first and second freewheel output.
 21. Amethod as recited in claim 19, wherein said step (5) further comprises:(a) driving a translational thrust system with the tail drive output.